Conventionally, electromagnetic actuators have been utilized as driving sources for acoustic elements such as loud speakers and the like because of its ease of handling. The electromagnetic actuator comprises a permanent magnet and a voice coil, and generates vibrations by the action of a magnetic circuit of a stator using a magnet. An electromagnetic speaker, in turn, generates sound by vibrations of a lowly rigid vibration plate of an organic film or the like which is fixed to a vibration section of an electromagnetic actuator.
Incidentally, the need for portable telephones and personal computers has increased in recent years, and associated therewith, small and power saving actuators have been increasingly demanded. However, since the electromagnetic actuator needs to apply a large current to the voice coil during its operation, it has a problem in power saving capabilities, and is not apt to a reduction in size and thickness from a structural viewpoint. In addition, the electromagnetic actuator must be electromagnetically shielded, when it is applied to an electronic device, in order to prevent evil influences due to leaking flux from the voice coil. From this aspect, the electromagnetic actuator is not either apt for use in small electronic devices such as portable telephones. Moreover, a reduction in size causes the voice coil to be made of a thinner wire, and a resulting increase in resistance of the wire can burn the voice coil as well.
In view of the problems mentioned above, piezo-electric actuators have been developed using a piezo-electric element such as piezo-electric ceramic or the like, which has such features as a small size and light weight, power saving capabilities, non-leaking flux and the like, for a driving source, as a thin vibration part which can substitute for the electromagnetic actuator. The piezo-electric actuator generates mechanical vibrations through motions of a piezo-electric element, and comprises, for example, a piezo-electric ceramic element (simply called a “piezo-electric element” as well) which is bonded to a seat.
The basic configuration of a piezo-electric actuator will be described with reference to FIGS. 1 and 2. FIG. 1 is a perspective view showing the configuration of a conventional piezo-electric actuator, and FIG. 2 is a cross-sectional view schematically showing how the piezo-electric actuator of FIG. 1 vibrates.
As shown in FIG. 1, piezo-electric actuator 550 comprises piezo-electric element 510 made of piezo-electric ceramics; seat 524 to which piezo-electric element 510 is fixed; and frame-shaped supporting member 527 for supporting the outer periphery of seat 524. As piezo-electric element 510 is applied with an AC voltage, piezo-electric element 510 performs expansion/contraction motions. As shown in FIG. 2, seat 524 deforms in a convex mode (indicated by solid lines) or deforms in a concave mode (indicated by broken lines) in accordance with the expansion/contraction motions. In this way, seat 524 vibrates in a vertical direction as viewed in the figures, with bonded area 524a serving as a fixed end and the center of the seat serving as a belly.
For reference, the piezo-electric actuator, though it is advantageous in a reduction in size ant thickness, exhibits poor performance as an acoustic element, in one aspect, as compared with the electromagnetic actuator. This is attributable to the piezo-electric element which is highly rigid and incapable of providing a sufficient average vibration amplitude as compared with the electromagnetic actuator. In other words, this is because, as the actuator has a small amplitude, sound pressure of an acoustic element is also reduced. On the other hand, JP-61-168971-A and JP-2000-140759-A disclose configurations which support the outer periphery of a seat by a beam which is relatively easy to deform in order to increase the amplitude of vibrations of an actuator.
Also, JP-2001-17917-A discloses, to the same effect, a technique for providing a large vibration amplitude with a plate spring which is created by slitting in a peripheral region of a seat along its circumference. JP-2001-339791-A also discloses a technique for widening a frequency characteristic by bonding the outer periphery of a seat with a supporting member through a curved supporting member.